This invention relates to curable formulations and cured compositions that are useful for producing coatings and in forming display films, in particular brightness enhancement films.
Brightness enhancement films have broad applications for flat panel displays in consumer electronics devices, such as laptop computers, televisions, projection displays, traffic signals, illuminated signs, camcorders, and the like. For example, in backlight displays, brightness enhancement films use prismatic structures replicated on the film surface to direct light along the viewing axis (that is, normal to the display), which enhances the brightness of the light viewed by the user of the display, and which allows the system to use less power to create a desired level of on-axis illumination. Typically, brightness enhancement films are made of plastic materials which are optically transparent and which comprise patterned surface features sometimes referred to as surface microstructure. One of the advantages of using plastic films for such applications is that, in principle, physical features of the films such as surface microstucture may be created using simple and effective techniques such as molding.
Optical materials and optical products, such as display films are advantageously prepared from high refractive index materials (typically materials having a refractive index of greater than or equal to about 1.6), such as polymerizable, high refractive index monomers and curable compositions containing such high refractive index monomers.
Useful high refractive index monomers include bromine-substituted aromatic (meth)acrylate monomers, such as, for example, those described in U.S. Pat. No. 4,578,445. High refractive index monomers are typically materials that exist in a crystalline or otherwise solid form under ambient conditions (ambient temperature) and have relatively high melting or softening points. Often such materials have melting or softening points significantly above ambient temperature. Typically, curable formulations must be substantially homogeneous in order to be processed into optical devices such as light management films. Thus, a curable formulation comprising monomers having melting points significantly above ambient temperature must either dissolve in the curable formulation, or the curable formulation must be heated to a temperature at which the curable formulation becomes substantially homogeneous. Heating a curable formulation during processing may significantly increase the cost and complexity of the processing. For example, it may be necessary that transfer lines used in a processing step be heated. Providing heated transfer lines increases the overall energy consumption of the process, and can increase the risk of transfer line failure (particularly at joints and valves) with the subsequent release of volatile organic compounds into the workplace and the greater environment. If uniform heating is not consistently maintained, the monomers having melting points significantly higher than ambient temperature may crystallize within the curable composition during processing, resulting in blockages in transfer lines or the production of a non-uniform and typically unusable product which must be disposed of. Further, maintaining the curable formulation at a temperature sufficient to maintain homogeneity may result in premature polymerization of the components of the curable formulation. These added process requirements and the negative consequences attendant thereupon increase the cost and complexity of producing a cured composition having a high refractive index.
Furthermore, a curable formulation designed for use in a brightness enhancing film produced using modem micro-replication processes must be a fluid which is typically free of solvent. Moreover, the fluid curable formulation must have the appropriate level of viscosity. Typically, the viscosity required of curable formulations used in microreplication processes is less than about 5,000 centipoises, preferably less than 1,500 centipoises at 25° C. In addition, a cured coating composition prepared from a curable formulation should have a relatively high refractive index, typically about 1.6 or higher; a glass transition temperature of less than about 100° C.; and suitable ductility, stiffness, dimensional stability, and adhesion strength to be useful in applications such as brightness enhancement films.
Therefore, it would be advantageous to discover substantially homogeneous curable formulations that are liquids at ambient temperature, that have a viscosity appropriate to the processing requirements of many different applications, including the production of optical films having replicated surface microstructures, and that can be processed as a liquid at temperatures at or near ambient temperature. Such curable formulations would be useful in the production of cured compositions for optical devices, such as high refractive index brightness enhancing films.